The invention relates to a fluorescent lamp comprising a glass discharge vessel in which a gas is present, which discharge vessel is provided with at least one tubular end portion having a longitudinal axis, which end portion is provided with a glass stem, while an exhaust tube extends axially in outward direction from the stem for the supply and/or discharge of gases during manufacture of the lamp, and an electrode extends axially in inward direction through the stem for maintaining a discharge in the discharge vessel during operation, the innermost end of the electrode being surrounded in radial direction by a shield for intercepting material sputtered off from the electrode, which shield is fastened on an elongate support which extends in inward direction from the stem.
An example of such a fluorescent lamp is the TL lamp of the Philips™ brand, with type no. F32T8 (also referred to as ALTO™ T8), a low-pressure mercury vapor discharge lamp which is commercially available.
Mercury is the primary component for the (efficient) generation of ultraviolet (UV) light in mercury vapor discharge lamps. A luminescent layer comprising a luminescent material (for example a fluorescent powder) is present on the inside wall of the discharge vessel for the conversion of UV into other wavelengths, for example into UV-A and UV-B for suntanning purposes (sun couch lamps), or visible radiation for general lighting purposes. The discharge vessel of a fluorescent lamp usually has a circular cross-section, and there are both elongate linear embodiments (TL tubes) and compact embodiments (energy-saving lamps). In the TL tube, said tubular end portions lie in one another's extended directions and form a long, straight tube, whereas in an energy-saving lamp they are interconnected by means of a bent tubular portion or a so-called bridge.
The fluorescent lamp is evacuated during manufacture through the glass exhaust tubes which are present at either end of the lamp. The desired gas mixture is subsequently introduced into the lamp through the same exhaust tubes, whereupon the exhaust tubes are closed by pinching or fusion.
During operation, a voltage is maintained between the electrodes, which are also present at the two ends of the lamp, so that a continuous discharge takes place and the mercury vapor emits the UV light mentioned above. The ends of the electrodes are radially surrounded each by a shield because small particles are regularly emitted by the electrodes during operation, which particles would end up on the inside wall of the discharge vessel. This is undesirable because it reduces the light output in situ, so that the lamp will have an uneven light output, which is why the particles are intercepted by the shield. The shield is fastened in the glass stem by means of a wire-type support.
The problem which may arise in such a fluorescent lamp is that, towards the end of lamp life when the electrodes have been partly exhausted, the discharge may continue between portions of the electrodes which were not designed for this purpose, during which the stem will be covered with metal particles originating from said portions of the electrodes. The shield, indeed, protects in radial directions only. As a result, the outer surface of the stem becomes conductive, with the result that the discharge applies itself thereto, and the stem becomes so hot that it softens and is deformed. The result is that the support with the shield, which is anchored in the stem, tilts and comes into contact with the electrode, and thus becomes part of this electrode electrically. In that case the shield will take over the electrode function. Owing to an unfavorable heat distribution, the wall of the discharge vessel may become excessively hot for a longer period as a result of this. It may eventually even happen that the shield sags against the glass discharge vessel, and that the latter is destroyed by the heat.